Derrick Bird's two sons are "mortified" at his horrific crimes but do not know what turned their "loving dad" into a mass killer.

Graeme, 28, and Jamie, 16, were "utterly devastated" by their father's death, and their thoughts were with the families of those killed and wounded in Wednesday's "tragic incident" in west Cumbria, they said in a statement.

As they remained bewildered over what triggered the killing spree that left 12 dead, including his twin brother David, and saw Bird, a 52-year-old taxi driver, take his own life, it emerged his surviving family feared they too could be targets.

His frail mother Mary, 90, who has cancer and is recovering from a stroke, learned of her son's 45-mile rampage on the lunchtime TV news on Wednesday while he was still at large being chased by police.

"She was horrified, she was astounded," said the Rev Jim Marshall, curate of St Michael's Church in the village of Lamplugh where Mrs Bird was at the time staying with her eldest son, Brian and his wife.

They locked their doors in fear of their own lives, he said, revealing a neighbour had alerted them to the incorrect news reports that Mrs Bird herself was one of the victims.

The whole family was suffering "dreadful sorrow" and "dreadful sadness", Marshall said. He added that one family member had said that "if they had the strength, they would love to go round each member of the families of those people who had been shot, killed and wounded in order to apologise for what has happened."

Insight into the terrible toll Bird's actions was taking on his own family came as members of the communities affected gathered for special services to commemorate the dead, which included local solicitor Kevin Commons, 60, and a taxi driver colleague Darren Rewcastle, 43.

The nine other victims were Garry Purdham, 31, Jane Robinson, 66, Michael Pike, 64, Kenneth Fishburn, 71, Jennifer Jackson, 68, and her husband James, 67, Susan Hughes,57, Isaac Dixon, 65, and Jamie Clarke, 23. They were all shot within an hour in apparently random attacks as Bird, armed with a shotgun and .22 calibre rifle, drove through Whitehaven, Egremont, Wilton, Gosforth, Seascale and finally to the hamlet of Boot, where his body was found in woodland at 1.40pm. Eleven people were wounded.

It has emerged that the Inland Revenue had been investigating Bird, who had spoken of his financial problems to friends prior to the killings. There is also speculation about whether any family feud over the will of the brothers' father Joe, who died 13 years ago, which reveals he gave David a £25,000 gift while still alive, could have contributed to Bird's state of mind. According to one report, Bird had been dumped recently by text by a Thai girlfriend he had met during frequent holidays to the country.

In their statement, Bird's sons said: "To us, he was the nicest man you could ever meet. He was a loving dad and recently became a grandfather.

"We would like to say that we do not know why our dad committed these horrific crimes. We are both mortified by these sad events.

"Dad was loving and cheerful character and was well-known throughout the local community and in the areas where he worked. He will be missed by us, his family and friends.

"We would also like to send our condolences to all the other families and people involved in this tragic incident. Our thoughts are with them".

Their uncle, Brian, said in a separate statement he and his family were "extremely saddened" and the loss of both his brothers, who were "very caring, family people", was "devastating".

Offering their condolences to the bereaved and injured, he said: "We appreciate what they are suffering at this time. We cannot offer any reason why Derrick took it upon himself to commit these crimes. We are in shock and dismay".

Marshall, said: "There was the Derrick they knew for 52 years, and then there was a new Derrick for a few hours last Wednesday. They can make no sense at all of what was behind it. There was never an open family rift."

He said the family had at one stage considered a joint funeral for the twins, but this was now unlikely to happen.

Details also emerged of a potential 13th victim who apparently survived only by chance.

Neighbours of Jason Carey, from Wilton, a scuba instructor with whom Bird was said to have rowed, said the taxi driver had banged on his door after killing others he apparently held grudges against. But Carey was still in bed and failed to answer his door, and Bird left.